To establish a seam on a fabric, today, there exists on the market a number of devices of different designs for performing lock stitches. On common home sewing machines an upper thread and a bottom thread on a bobbin in cooperation with a needle are used, in a known way, to bring the upper thread to perform a lock stitch on the fabric, which is sewn on the sewing machine.
A correct relation between the length of the upper thread and the length of the bottom thread of a stitch is desirable to accomplish a seam that looks decorative and holds a high quality. The proportion between the length of the upper thread and the bottom thread of each stitch depends on the relation between the tension of the upper thread and the bottom thread, respectively, during the forming of a knot that is made by upper thread and bottom thread and which constitutes a lock for a stitch in the seam.
To obtain the desired quality of a stitch, it is desirable that the knot of a stitch can securely be placed at the desired location in relation to the fabric. Usually, an optimal location of the knot is in the middle of the fabric as seen in a cross section of the extension of the fabric.
In prior art it is known to automatically adjust the present thread tension of the upper thread based on thread consumption of former stitches of the seam. Such a device is disclosed, as an example, in document U.S. Pat. No. 6,012,405. In this device the real thread consumption of the upper thread is measured be means of a decoder for consumed thread length after a completed stitch, whereby this information about real thread consumption for an already performed stitch is used to adjust the thread tension of a subsequent stitch to accomplish a correct relation of thread lengths between upper thread and bottom thread. This and other similar solutions presupposes that the amount of upper thread required for a present stitch is known in advance.
Document U.S. Pat. No. 4,967,679 discloses a solution to attain automatic control of thread feed, wherein a sewing machine for straight seams utilizes thread out portioning in accordance with a requisite amount of upper thread, and which adjusts the thread tension of the upper thread at zigzag sewing. The thread out portioning of this solution is mechanically driven, wherein rolls which drive the thread are rotated synchronically with the driving mechanical members of the sewing machine. At zigzag sewing the thread tension is electrically set according to a manually predetermined value.
In this described solution the consumption of upper thread per stitch may not be predetermined for other types of seams than for straight seams. The thread out portioning is effected in dependence of a cyclic movement on a mechanical member of the sewing machine. The selected mechanical member executes the same cyclic movement also for other types of seams, e. g. at zigzag seams, whereby thread may not be fed by means of thread out portioning controlled by said mechanical members other than for straight seams.
However, the possibility to predetermine the thread consumption of upper thread consumption for a subsequent stitch is known. Such a determination takes into consideration, without being limited to these factors, stitch length, the thickness of the fabric, angular deviation between a present and a subsequent stitch, stitch speed and other parameters set by an operator. Such a determination of thread consumption is shown in the publication U.S. Pat. No. 6,012,405, a publication, the content of which in its entirety is incorporated into this description by reference.
Earlier, it has been problematic to use thread out portioning as an alternative to friction braking and to be able to freely switch between these, when in both cases the control unit of the sewing machine shall fulfil the demand to control the out portioning of upper thread and the brake force at friction braking respectively.
The present invention provides a device and a method to render a free choice of thread feed possible by means of a selection of thread out portioning or friction braking for all types of seams performed by the sewing machine.